The Caliciviridae is a family of positive-strand RNA viruses and consists of five genera designated: (1) Norovirus (with species Norwalk virus); (2) Sapovirus (with species Sapporo virus); (3) Vesivirus (with species, feline calicivirus and vesicular exanthema of swine virus); (4) Lagovirus (with species rabbit hemorrhagic disease virus and European brown hare syndrome virus) and (5) Nebovirus (with species Newbury-2 virus). The diseases caused by caliciviruses vary, according to the virus and its host species. Members of the Caliciviridae causing diarrheal disease in humans belong to the genera Norovirus and Sapovirus. The Caliciviruses Section (CS) in LID conducts research related to caliciviruses, with an emphasis on the noroviruses because of their predominant role in human diarrheal disease. Noroviruses are genetically-diverse and most human norovirus pathogens belong to either Genogroup I (GI) or Genogroup II (GII), with GII.4 as the predominant genotype. We continued an epidemiological investigation of noroviruses in immunocompromised patients enrolled in research protocols at the NIH Clinical Research Center. A multiplex assay for the detection of bacterial, viral, and parasitic gastrointestinal pathogens became available for use in the Clinical Center near the end of 2015. In collaboration with epidemiologists in NIAID we explored the relative contribution of viral pathogens to diarrheal illness since its employment. Consistent with our earlier studies, noroviruses remained the predominant viral pathogen detected, associated with infection in approximately 12% of patients with diarrhea. Astroviruses and sapoviruses were detected in our patient cohort for the first time. Rotaviruses, now controlled with successful vaccines first developed in NIAID, were rarely detected in immunocompromised patients with diarrhea. We are continuing our epidemiological survey of these patients to understand the role of enteric viruses and how noroviruses adapt to become chronic. We have expanded the use of pathology in our studies, aiming to identify and characterize the host target cells of the virus in the enteric tract. In tandem, we are using human enteroid 3-dimensional cell culture technology and single cell RNA sequencing to understand the replication of human norovirus in permissive target cells. We aim to identify an effective therapeutic option for these patients and design a clinical trial to evaluate it. High throughput technologies with the cytopathic murine norovirus model allowed additional new discoveries this year. We performed the first direct comparison of murine norovirus replication in primary macrophage versus a continuous macrophage cell line (RAW264.7). Although the effect of viral replication on a number of cellular pathways overlapped, we discovered important differences. For example, the effects of norovirus replication on cell cycle pathways were pronounced in the continuous cell line, but negligible in primary macrophage. Our work showed that the selection of cells for transcriptomic analysis should be considered carefully. depending on the hypothesis under investigation. Human epithelial cell lines bearing the murine norovirus receptor were constructed so that the inhibitory effects of antiviral drugs and interferon lambda on norovirus replication could be studied in the context of human cellular pathways.